I'm a collector. I'll keep anything and everything. I'll keep a record, a receipt, a photo, a magazine, a card, a phone number long since disconnected. Everything. It's a bit of a burden, as any collector knows. At some point, the boxes own you. Moving is a horrific experience. And actually trying to find something in these towers of boxes is always a futile endeavour. But there they sit, the boxes, and every once in a while this pack-rat mentality can be a little bit profound. Like today, randomly coming across a dusty container marked: "Pearl Jam – Stuff/90s." Saved for a rainy day. And here it is. Raining.

The timing is interesting. We've been in the editing room for about a year, working on Pearl Jam Twenty, a film celebrating this truly great Seattle band's first 20 years. I thought we'd searched every corner and crevice, called in every random news report and interview, transferred every essential piece of Super 8 from the band's visual archive – and now I find more stuff. It's too late for it to make the movie, though.

The box is filled with tapes and bootlegs and music and notebooks. Digging deeper, there's a carefully adorned aeroplane sickbag with these words on it, drawn in a sparkly pen: "From Eddie." In it is a tape of early demos. I remember this tape. We were making the 1992 movie Singles, about twentysomethings in grunge-era Seattle, and we'd given the band jobs in and around the movie. Jeff Ament, the bass player, worked in the art department, and I borrowed much of his stuff to decorate the apartments in the movie. Jeff had the greatest synthesis of art, sports and film on his walls: from David Lynch to obscure metal, from Kings X to the SuperSonics basketball team.

That mix of all that was important artistically and soulfully had a big effect on the ethic that would spawn Pearl Jam. Art is everything, Jeff seemed to be saying with his life choices. Jeff's taste, and that of his musical cohort guitarist, Stone Gossard, was inspiring, and genre-bending. It was OK to like disco, hard rock, Kiss, Queen and the blues. It all came out in their music, in the dark promise of their early band, Green River.

And look, here's a photo of the Seattle sky, late 1980s. A dark blue horizon speckled with bright northern stars – that's what the music sounded like then: indigo, flashy, melodramatic and fun. With a sky like that, it's hard not to look up. And it's hard not to feel it, even locked away in a garage, slashing through chords and looking for the right mix of influences. It doesn't always rain in Seattle, but certainly music from the northwest has its roots in players who stay indoors and play and listen and listen and play. A lot. Thus, the tradition of musicians who have the time to feel it and get it right.

Even from the beginning, there was a generosity about Pearl Jam. Jeff, Stone, guitarist Mike McCready and, later, singer Eddie Vedder all had an openness to music and the world, and an almost superstitious attention to the details of how this band would be different. The band itself sprang from a miracle. Stone and Jeff had been playing with a local luminary, Andrew Wood, a singer and writer of monumental charisma and talent. When Wood died from a heroin overdose on the eve of his band Mother Love Bone's first big tour, the loss was beyond seismic.

But when a demo tape for Stone and Jeff's next musical project went forth into the world and found its way to a young San Diego surfer who connected immediately, no one could believe lightning could strike again, much less that fast. Before long, the shy surfer Eddie was sitting among us, staring down behind a sheet of wavy brown hair, barely speaking but trying to fit in. And every once in a while, he would pull the hair out from in front of him, and look at us with those flashing, mischievous eyes and … you knew. This was a guy who shared the same high-stakes love of all that was possible.

One night, sitting cross-legged at a friend's house listening to Neil Young tapes, Eddie told me how he'd discovered his biological dad was actually a family friend who'd passed away. It was almost a confession about where some of the finely etched anger in his songs was coming from. But mostly we talked about Pete Townshend of the Who. Besides the Who, then unchallenged as the greatest band in rock, we both loved the Rolling Stone journalism of Townshend. Townshend still is rock's most articulate spokesman, surely the best rock journalist of them all, because he wrote from the inside out. He wrote about a healing belief in the power of rock. There was nothing jaded about his relationship with music. His love of rock was almost religious, and so fervent that when the famous anarchist-politician Abbie Hoffman tried to grab a microphone and say a few words in the middle of the Who's performance at Woodstock, Townshend famously swatted him from the stage with his guitar.

A captain on a pontoon ship

Hearing Eddie's recording on that first demo, I felt that same fervour. The music was a sanctuary, a place where something more than just rock might occur. Like the others, Eddie was a fan, and any fan knew that, handled correctly, music could take you rushing down the current to places you'd never imagined. That is, if you cared enough. And concentrated enough. This would be an experiment in alchemy. "I just want to play," I remember Eddie saying. "Just want to keep playing."

Message: No bullshit need interrupt.

The audience for the band's first show was filled mostly with anxious and hopeful supporters. Eddie was nervous, swaying backward and forward, side to side, holding on to the mic stand like a young captain steering a pontoon ship. The songs are what stood out: confident, honest and memorable. There was one that towered over the rest, Release, written from way down deep, filled with open-hearted emotion. It reached people instantly. That galvanising moment, with audience and band coming together, set the stage for all that would follow. They changed their name to Pearl Jam. Soon the hurricane of success would touch down.

The songs guided them through the first album. The songs moved people, showed them a new style of commitment that had been increasingly absent in rock, and sure enough, in shockingly short order, the group's popularity found them living under the white heat of scrutiny. They proceeded in their own fashion, with ever-changing shows, and ever-changing rhythms, always staying close to their original instincts. Their first manager, Kelly Curtis, is still their manager. He, too, guides the band with a simple credo: stay true to the music.

Keep playing but grow smaller

I open a second box, marked: "PJ 2000s." Here we see the second act of a band who survived that first tidal wave of popularity. One of the surprises is that their stubbornness as artists sometimes had its media detractors. At times, even other bands found Pearl Jam's steadfast desire to keep playing but grow smaller confounding. The band would swear off videos and most interviews for years. That stubborn muscle was always there in them, though, all the way back to Eddie refusing to release Black, from Ten, as a single, even when all the record company bigwigs and radio programmers were bombarding the band with demands for it.

To anybody who lost track of Pearl Jam in the 1990s or 2000s, a surprise was awaiting if they dropped in on a live show. There they'd find a passionate arena filled with followers, keyed to nearly every lyric, many having followed the band from show to show. It was like what Jeff had seen in the 1990s, checking out Grateful Dead shows. He was blown away by the lack of distance between fan and band, and "the fact they would get the greatest response from playing an obscure song they hadn't played in 17 years. I remember looking at that and thinking, 'That's success.'"

Thanks to the band members – for opening up their homes, lives and archives to us. They are friendly but private guys, as their fans know well. So the very act of releasing this bounty of information and souvenirs, clues and myths and facts, is all part of the group's ongoing sense of reinvention. Some might have never expected such an open-book approach. But then again, as we say in Pearl Jam Twenty, they have become over time the most dependably unpredictable band in rock.

Turn it up!

This is an edited extract from Pearl Jam Twenty, published by Atlantic Books. Cameron Crowe's film is in UK cinemas on 20 September, for one night only only. Details: pj20.com/screenings